Power detectors are used in radio applications for many different purposes. For example, an envelope detector can be used as a simple power meter showing transmitted output power, received signal strength and measured standing wave ratios in radios and service instruments. Envelope detectors typically include a diode and capacitor or a four diode ring rectifier. The diode included in an envelope detector is typically implemented as a junction diode (such as a Schottky diode) using standard CMOS process technology. However, junction diodes have a large forward voltage drop and poorly controlled operating parameters. A power detector can also be constructed from one or more transistors by operating each transistor in the non-linear region. For example, an unbalanced pair of CMOS transistor devices can be used for RMS-detection (Root Mean Square) by utilizing the quadratic operating characteristic of the transistors. RSSI (Receiver Signal Strength Indicator) detectors, often used in Bluetooth and WLAN can be combined with an unbalanced transistor pair and a saturated amplifier to increase dynamic range. For example, a softly saturated amplifier together with an unbalanced pair can be designed for small offset voltage.
Power detectors have specific requirements such as dynamic range, signal level, type of value detected (peak, RMS, etc.), frequency range and temperature dependence. Diode-capacitor and single transistor-capacitor power detectors have very small dynamic range, limiting their usefulness. Also, single transistor power detectors are extremely temperature sensitive. Unbalanced pair power detectors are less sensitive to temperature, but have a fundamental built in DC-offset voltage that limits dynamic range. Softly saturated amplifier-based power detectors have a broader dynamic range, but consume more area. Softly saturated amplifier-based power detectors also have a relatively small upper frequency limit. Thus, a power detector that has relatively low DC offset, high dynamic range, small frequency and temperature dependence and low flicker noise is highly desirable.